1. FIELD OF THE INVENTION
The present invention concerns an exhaust gas purifying catalyst which decomposes, into non-toxic gases, nitrogen oxides discharged from mobile internal combustion engines used, for example, in diesel cars, stationary internal combustion engines used, for example, in cogeneration systems and various industrial furnaces such as boilers.
2. DESCRIPTION OF THE RELATED ART
Generally, exhaust gases discharged from automobiles, stationary internal combustion engines and various industrial furnaces contain a great amount of nitrogen oxides represented by NO and NO.sub.2 (NO.sub.x).
It is said that such NO.sub.x not only causes photochemical smog but also induces disease in man's respiratory organs. As a method of decreasing NO.sub.x, an exhaust gas processing technique using a so-called ternary catalyst system is established that eliminates NO.sub.x by reduction in an exhaust gas of low oxygen content such as that from gasoline cars by using a reducing agent such as carbon monoxide or hydrocarbon.
On the other hand, in the case of an exhaust gas containing a great amount of oxygen such as that discharged from large-scaled stationary exhaust emission sources such as boilers, a selective N.sub.x reduction process for decreasing the amount of NO.sub.x by external addition of ammonia is now under actual operation, which produces some effect.
However, the former method is applicable only to an exhaust gas from a gasoline engine in which the oxygen concentration is extremely low whereas the latter method is difficult to use in small-sized stationary exhaust emission sources or mobile exhaust emission sources from a standpoint of handling because ammonia is used.
In view of the above, various methods have been studied for using hydrogen, carbon monoxides or various hydrocarbons as reducing agents other than ammonia, but most of them have the drawback that they are a non-selective catalytic reduction process which can eliminate nitrogen oxides only after oxygen in the exhaust gas has been consumed completely.
Although the following methods have been proposed so far as a novel selective catalytic reduction process capable of overcoming such a drawback ( methods of selectively reducing and eliminating nitrogen oxides even under the coexistence of oxygen), none of them can provide quite satisfactory results.
Published unexamined Japanese patent application Hei 2-149317 proposes a method of using either (1) a catalyst comprising a hydrogen type mordenite or clinoptilolite or (2) a catalyst comprising a hydrogen type mordenite or clinoptilolite supporting a metal such as Cu, Cr, Mn, Fe and Ni, and bringing exhaust smoke containing oxygen resulting from combustion of various fuel into contact with the above-mentioned catalyst under the coexistence of an organic compound thereby eliminating nitrogen oxides in the exhaust smoke. ,
According to this method, a denitrating ratio of 30 to 60% is achieved under the conditions of a reaction temperature of 300.degree. to 600.degree. C. and a gas hourly space velocity (GHSV) of 1200 h.sup.-1, but a denitrating effect under a high GHSV condition, i.e., a condition approximate to that of practical use is not clear. Further, no literature gives a description of the aging of the catalytic activity and specified life of the catalyst. Further, since the catalyst is evaluated using a pseudo exhaust gas containing no SO.sub.x, the resistance of the catalyst to SO.sub.x is uncertain.
Published unexamined Japanese patent application Hei 1-130735 proposes a method of using a catalyst in which a zeolite ion-exchanged with a transition metal (Cu, Co, Ni, Fe, Mg, Mn or the like) is carried on a fire resistant support, and purifying nitrogen oxides even in an oxidative atmosphere.
This is a method of purifying nitrogen oxides in an exhaust gas from a gasoline engine at high efficiency even in a lean air/fuel ratio region, in which the oxygen concentration in the exhaust gas is about 3% at most. Accordingly, it is uncertain whether or not nitrogen oxides can be selectively denitrated by reduction also in an exhaust gas such as that under a lean condition of high air fuel ratio in a gasoline or from a diesel engine in which the oxygen concentration is from 5 to 10%. Also in the examples, the NO.sub.x eliminating ratio tends to be lowered greatly along with an increase in the oxygen concentration and the permanence thereof is not clear.
Published unexamined Japanese patent application Sho 63-283727 proposes a method of using a catalyst in which a metal such as Cu, V, Mn, Fe or Cr is supported on a hydrophobic zeolite with a SiO.sub.2 /Al.sub.2 O.sub.3 ratio of 15 or more, and decreasing nitrogen oxides in an oxygen containing exhaust gas from an internal combustion engine in the presence of carbon monoxide and one kind or more of hydrocarbons.
In this method, the denitrating ratio is decreased to as low a value as 4 to 26% in the case of using a zeolite catalyst supporting a metal other than copper. On the other hand, in the case of using a copper-zeolite catalyst, a permanence is not always clear though relatively high activity is obtained. The oxygen concentration in the exhaust gas shown in the examples is 1.6% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher, for example, as in an exhaust gas under a lean condition of high air fuel ratio in a gasoline or from a diesel engine.
Published unexamined Japanese patent application Sho 63-100919 proposes a method of using a catalyst in which copper is supported on a porous support of alumina, silica or zeolite, and eliminating nitrogen oxides in an exhaust gas containing oxygen in the presence of a hydrocarbon.
In this method, the denitrating ratio is from 10 to 25% and no high denitrating activity is obtainable. Further, since the catalyst contains copper, there is the problem that the copper ingredient is readily poisoned by SO.sub.x. Further, the oxygen concentration in the exhaust gas shown in the examples is 2.1% and it is uncertain whether or not nitrogen oxides can also be reduced selectively for denitration if the oxygen concentration is higher. Also, the permanence thereof is uncertain.
Published unexamined Japanese patent application Hei4-4045 discloses an exhaust gas treatment catalyst in which copper and certain metal(s) coexisting with the crystalline silicate defined by the given formula and exhibits the specific X-ray diffraction pattern.
This catalyst contains one or more of 27 different metals coexisting with the crystalline silicate. However, the difference in advantages or the comparative merits resulting from the metal(s) used is unclear. The preferable ranges of contents per carrier of 100 parts by weight are relatively wide: 0.2 to 8 and 0.1 to 6 parts by weight for copper and other metals, respectively. In addition, the optimum content for each of the 27 metals is unclear. It is assumed that the amount of the given metal(s) coexisting depends on that of the copper supported. However, only the preferable amount of copper is defined for this catalyst and the optimum amount of the given metal(s) relative to the copper, i.e., [given metal(s)/copper (molar ratio)] is not clear.
In this respect, the present invention is directed to providing an exhaust gas purification catalyst of high resistance which allows highly effective purification of nitric oxide into a nontoxic gas, even for an exhaust gas containing oxygen at a high concentration.